Add generics to extrinsics crate

[smiasojed]

Summary

Related #1168

• y/n | Does it introduce breaking changes?
• y/n | Is it dependent on the specific version of ink or pallet-contracts?

Add generics to extrinisics crate

Description

The PR removes PolkadotConfig and DefaultEnvironment from the extrinsics crate and makes use of generics for Hash, Account and Balance.

Checklist before requesting a review

• My code follows the style guidelines of this project
• I have added an entry to CHANGELOG.md
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• Any dependent changes have been merged and published in downstream modules

[cmichi]

Thanks, Sebastian. Looks good!

[cmichi]

Please also add a test for the failure case .is_err().

[ascjones]

I'd like to see a bit more in the PR description to explain the change and how it is an intermediate step towards #1463.